Effective voiding control of QFN via solder mask patterning

Voiding under QFNs is a major challenge in the electronics industry. However, voiding can be suppressed by improving venting accessability on the thermal pad by using solder mask dividing strips. Venting accessability is defined as the perimeter length per unit area of the metal pad. Regardless of the shape and the number of subpads, increasing venting accessability results in a decrease in the total number of voids, the largest voids, and discontinuity. Voiding caused by peripheral vias is comparable to voids caused by hidden vias. Voiding increases with decreasing print coverage and is attributed to insufficient solder. Voiding also decreases with an increase in the number of thermal vias. This phenomenon is attributed to volatiles bleeding through the small voids around the thermal via.